Eating feces is usually a sign your dog is trying to get enzymes. Sometimes it's just a habit and something they like to do. But usually there's some nutrients in the waste they're trying to make up for. See, living creatures were designed to eat fresh, raw food (some more than others) and when they do so they receive the enzymes present in the food and this helps them digest, break the food down, and actually benefit from the food. When one is fed processed, cooked food day in and day out their body has to pull from their resources of enzymes they're supplied with -- but these resources are limited and eventually will dwindle down so your dog will be eating, but most likely not getting anywhere near the full amount of nutrition the food has to offer. You really don't want to use up your supply as you don't get new ones unless it's from the food your eating.
Kibble has added vitamins and such, but synthetic vitamins don't really do the job as they contain just one essential part of the nutrient they're trying to make up for, whereas in the natural state many parts make up that nutrient and you can't derive nearly as much value without all parts. They can usually get a human by until they reach around 50 so you're looking at what? around seven years old before problems start really setting in? Sounds like it would be around the age most dogs start having issues these days, sometimes much younger if the bitch has suffered this way and passed that on. As you can see with humans, we're living shorter lives than the previous generation. Sad though considering how much more knowledge we have access to than previous generations.
If you're going to feed your dog processed foods, which I wouldn't recommend, it's vital you at least add enzymes and make sure they're animal-based enzymes, not plant. I would recommend Dr. Mercola's offhand, but if your dog has a pork allergy, they do use pork for the bile. It can take a few years, depending on the nutrient deficiency, for a human to show the signs of not properly digesting/breaking down nutrients. In fact, I had this issue and it didn't start becoming blatantly obvious until about 28 years into life so in a dog that would be around 4-5 you'd start seeing issue.
A lot of dog foods on the market claim to add enzymes, but they're very finicky and I wouldn't trust that. You will be amazed by the reduction in stools -- trust me! And please keep in mind, no matter what she eats, if the enzymes aren't there and her supply is dwindling, she won't get the nutrients of any food. Most people don't understand the body has to be in the right state to convert nutrients and use them. It doesn't just happen because we've eaten... we can eat all day long but if the balance isn't there, the bacteria and the enzymes and the process doesn't go as it should (even something as simple as if you don't drink enough water with a meal) the animal (or human) won't be able to utilize those nutrients and they'll either sit in the stomach and ferment, which then causes things like yeast infections and parasites to take over, or they'll eventually the malabsorption will catch up with them and lead to other serious health issues, or they'll be removed through waste, normally diarrhea. This is what causes flatulence in animals/humans. They're eating something, most likely process carbs, that their body can't digest for whatever reason so it's sitting in the lower part of the gut, fermenting. If you're eating something and getting gas from it, that means your body didn't digest it properly=you're not getting nutrients from it and it's harming the gut by throwing it off balance and allowing fermented things to sit around and draw bad bacteria and then killing off all the bad bacteria. There is a colony of live bacteria in the gut, and if you don't keep everything inline and eat what the body is MEANT to digest, the whole body suffers.
She is fed quality raw, organic veggies and treats, plus digestive enzymes and vitamins added to her diet. Our other dog who eats high quality kibble and also gets digestive enzymes and vitamins is always a poop eater, so definitely not always a nutrient issue.